1. Field of the Invention
The invention relates to compositions comprising organosilicon compounds having branched and/or cyclic, preferably saturated alkyl groups attached directly to silicon, and to their use as defoamers.
2. Description of the Related Art
In many liquid systems, especially aqueous systems which include surface-active compounds, sometimes as unwanted constituents, it is possible for foaming problems to occur if these systems are contacted, more or less intensively, with gaseous substances. Examples include the gassing (generally, aeration) of wastewaters, during the intensive agitation of liquids, during distillation, during washing or dyeing operations, or during dispensing processes, as but limited examples. The resulting foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have proven particularly appropriate.
Siloxane-based defoamers may be prepared as disclosed by DE-B 15 19 987, by heating hydrophilic silica in polydimethylsiloxanes. Addition of basic catalysts during the treatment process allows the effectiveness of such defoamers to be improved, as disclosed in DE-A 17 69 940, for instance. An alternative is to disperse hydrophobicized silica in a polydimethylsiloxane, as disclosed, for example, in DE-A 29 25 722. Nevertheless, the effectiveness of the resulting defoamers is still in need of improvement. Thus, for example, U.S. Pat. No. 4,145,308 describes a defoamer preparation which in addition to a polydiorganosiloxane and silica, further comprises a copolymer made up of (CH3)3SiO1/2 and SiO2 units. Copolymers made up of (CH3)3SiO1/2 and SiO2 units are also said to be advantageous in combination with siloxanes which carry terminal long chain alkyl groups, as described, for example, in EP-A 301 531. The use of partly crosslinked polydimethylsiloxanes which are in some cases already rubberlike is said to contribute to increasing the defoamer effect. On this point, reference may be made, for example, to U.S. Pat. No. 2,632,736, EP-A 273 448 and EP-A 434 060. These products, however, though, are generally of very high viscosity and are difficult to handle or to process further.
Generally polysiloxanes having methyl groups, such as polydimethylsiloxanes, are used in defoamers. Although polymers with a range of other aliphatic or aromatic hydrocarbon groups on silicon are known and are also proposed in numerous patents for the preparation of defoamers, there are few indications that by selecting from among such substituents, it is possible to achieve a substantial improvement in the defoaming effect. Frequently, the aim of introducing long alkyl groups or polyether substituents is to improve the compatibility with mineral oils that may be present in defoamer compositions, or to prevent silicone defects in coatings, such as “fisheyes”, for example. Thus EP-A 121 210 recommends the use of polysiloxanes which carry alkyl groups having 6-30 carbon atoms whereby the fraction of carbon in the form of the CH2 group is 30%-70%, in combination with mineral oil. In the examples, mention is made in particular of polysiloxanes having octadecyl groups. JP-A 60173068 recommends siloxanes having octyl groups and polyether groups as defoamers in aqueous printing inks. Siloxanes having alkyl groups with more than 30 carbon atoms in combination with amino siloxanes are said by U.S. Pat. No. 4,584,125 to be advantageous for an antifoam effect, especially when the fraction of siloxane units carrying these moieties is around 5%.
In strongly foaming, surfactant-rich systems, however, the defoamer formulations prepared in accordance with the prior art do not always have a sufficiently long-lasting effectiveness or else, owing to high viscosity due to the degree of branching or crosslinking involved, are difficult to handle.